Al-X / AlOx / Al junctions

Al-X / AlOx / Al junctionsEdit

step 1: bilayer

When spin on full wafer:

- spin MAA8.5 EL10 (batch , exp. ) @ 2000rpm for 60"
- bake hot plate setpoint 180°C, 5'
- spin PMMA A6 (batch , exp. ) @ 6000rpm for 60"
- bake hot plate setpoint 180°C, 15'

The thickness was measured with two techniques:

First, we define lines with SEM, develop, cleave, sputter and observe under angle
Tihs gives the following values (statistics on 5 - 6 samples, with 30 lines on each)
Drawback: the resist is modified by the SEM observation, even though it is sputtered, and we make fast images.
MAA = 530 +/- 20nm
PMMA = 240 +/- 20nm

Second, we infer the bilayer thickness from double angle displacement (first evaep @ 0°, second evap at 35° for instance). With a statistic over 4 samples, with 4 patterns on each, we obtain:
Drawback: the mask is closing with the first evaporation. The mask might be lifted up or down.
MAA = 670 +/- 60nm
PMMA =  +/- nm

When spin on single 5x5mm chip:

- spin MAA8.5 EL10 (batch , exp. ) @ 2000rpm for 45" + 10" @ 8000rpm (acc. 4000rpm/s)
- bake hot plate setpoint 180°C, 5'
- spin PMMA A6 (batch , exp. ) @ 5000rpm for 45" + 10" @ 8000rpm (acc. 4000rpm/s)
- bake hot plate setpoint 180°C, 15'

comparable thicknesses, relatively more uniform spin than with standard recipe.
Measured on one sample, at ellipsometer,
MAA = 660nm
PMMA = 320nm @ 4000rpm (no measure @ 5k or 6k)

step 2: exposure

Precise exposure pattern was quite long to optimize.
GUIDELINES:

• Have a sufficient dose for the connections. Standard dose = 300µC/cm² for 30keV

spot 1: dose x 1.4 (in position list)
spot 4: dose x 1.2 (in position list)
spot 7: dose x 1.8 (in position list). This compensates for drift in current along exposure. Do critical patterns first (ie alignement cross, or small arms)

• Have a large overlap between patterns

on x2000 field, overlap is at least 
between x2000 spot 1 and x1000 spot 4, overlap is 
between x1000 spot 4 and x32 spot 7, overlap (tolerance) is 3µm

Note there is significant proximity effect of large connecting areas onto the small patterns (eg the undercuts), over few µms.
 

step 3: development

Development will affect the subsequent resist residues and undercut.
Although no recipe has been shown to really impede resist residues, we found that rinsing using a mix of ethanol and IPA (1:1) increases the undercut, which might also reduce the residual resist atomic layer on the substrate.

No comparative measurements were done on the undercuts. (TODO)

dev: MIBK - IPA (1:3) 1'
rinse: IPA - ethanol (1:1) 1'
rinse 2: IPA 15"

step 4: evaporation

- ion mill 3mA 500V, 20" @ 0°
optional - Ti pump ([email protected]/s on backside of holder) -> P_ch = 4e-8
- Al 30nm @ 1nm/s, 0°, P_ev = mb
- ox 5'@ XXXmb
( when double oxidation is required
- Al 0.6nm @ 0.1nm/s, 0° (note each time the actual value at shutter close)
- ox 5' @ XXXmb
)
optional - Ti pump -> P_ch = 8e-8
- Al 60nm @ 1nm/s, +35°, P_ev = 4e-7mb

This 0° / +35° with the above bilayer gives ~550nm displacement (from top of bilayer), and 450nm displacement including masking by the PMMA hanging mask (so from bottom of PMMA).
This indicates: 785nm total thickness, 640nm MAA thickness

Note when using 40° angle, one has 600nm displacement from top of bilayer, 500nm from bottom of PMMA. 
-> 600nm MAA, 715nm total

step 5: lift-off

Warm (65°C) PG remover for about 30 minutes.
NO ULTRA SOUND! Use pipette to finish lift.
Rinse ODI

step 6: junction stabilization to ageing

• Asher O₂, 2' @ 100W, 200µb to remove resist residues and stabilize junctions resistances.
  Effect is systematic increase by about 10% after ashing
• 1 min @ 100°C on hot plate.
  Effect is to lower dispersion on junctions values, and is typically similar to asher (increase by 10%)

step 7: probe station measurement

Preferably use 4W measurements that are more accurate. The leads are typically 60ohm each.
Fill the table below with measurements

left: after SEM observation
right: + ozone plasma 2', 200µb, 100W
red: got shunted